POMs are a unique class of inorganic metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge which is balanced by cations that are usually external to the cage, and may also contain centrally located heteroatom(s) surrounded by the cage framework. Generally, suitable heteroatoms include Group 13-16 elements such as phosphorus, antimony, silicon and boron. The framework of POMs comprises a plurality of metal atoms (addenda), which can be the same or different, bonded to oxygen atoms. Up to now the framework metal is substantially limited to a few elements including transition metals from Group 5 and Group 6 in their high oxidation states, e.g. tungsten (VI), molybdenum (VI), vanadium (V), niobium (V) and tantalum (V).
The first example in the POM family is the so-called Keggin anion [XM12O40]3− with X being a heteroatom selected from a wide variety of elements such as P and M being a Group 5 or Group 6 metal such as Mo or W. These anions consist of an assembly of corner- and edge-shared MO6 octahedra of the metals of Groups 5 or 6 around a central XO4 tetrahedron.
In the past, there have been increasing efforts towards the modification of polyoxoanions with various organic and/or transition metal complex moieties with the aim of generating new catalyst systems as well as functional materials with interesting optical, electronic and magnetic properties. In particular, transition metal-substituted polyoxometalates (TMSPs) have attracted continuously growing attention as they can be rationally modified on the molecular level including size, shape, charge density, acidity, redox states, stability, solubility etc. To date many 3d transition metal containing POMs are known, but only a few POMs containing 4d and 5d metals are known. However, the introduction in a POM of 4d and 5d metals appears to be of fundamental interest. Especially, ruthenium containing POMs are of interest because they are thermally and oxidatively stable and possess highly attractive catalytic properties.
For example, Neumann et al. describe the preparation of ruthenium-substituted “sandwich” type polyoxometalate [WZnRu2(OH)(H2O)(ZnW9O34)2]11− as well as its ability to catalyze the oxidation of alkanes and alkenes using hydrogen peroxide and molecular oxygen as the oxygen donor (see: Angew. Chem. Int. Ed. Engl. 1995, 34, 1587; Inorg. Chem. 1995, 34, 5753; J. Am. Chem. Soc. 1998, 120, 11969 and Nature, 1997, 388, 353-355). Moreover, Pope et al., J. Am. Chem. Soc. 1992, 114, 2932, disclose the synthesis of the cesium salt of [PW11O39Ru(H2O)]4+ and characterize its oxygen atom transfer reactivity. In all these syntheses, RuCl3.nH2O or [Ru(H2O)6](C7H7SO3)2 are used as ruthenium sources.
Further, Ru3+-substituted silicotungstates such as [SiW11O39RuIII(H2O)]5−, its use as catalyst for the oxidation of various alkanes and alcohols and its dimerization to the μ-oxo-bridged dimer [{SiW11O39Rum}2O]n− have been described (Mizuno et al., New J. Chem., 2002, 26, 972-974; Sadakane et al., Dalton Trans., 2003, 659-664; Sadakane et al., Dalton Trans., 2006, 4271-4276; Sadakane et al. Dalton Trans., 2007, 2833-2838).
Nomiya et al., J. Chem. Soc., Dalton Trans. 2001, 1506, discuss the difficulty of making pure Ru-containing POMs and the non-reproducibility of some reported Ru-substituted polyanions.
Recently, the dimethyl sulfoxide (dmso) complex cis-Ru(dmso)4Cl2 has become a popular ruthenium(II) source for the synthesis of Ru-substituted POMs. For example, Kortz et al. disclose the preparation and structural characterization of [HW9O33Ru2(dmso)6]7−, [Ru(dmso)3(H2O)XWO39]6− (X═Ge, Si) and [HXW7O28Ru(dmso)3]6− (X═P, As) (see: Chem. Commun. 2004, 1420; J. Chem. Soc., Dalton Trans. 2004, 3184; and Chem. Commun. 2005, 3962).
However, up to now these RuII(dmso)3-based anions have not turned out to be very useful for homogeneous or heterogeneous catalytic applications.
In addition, the use of a RuII(arene) containing source for the synthesis of Ru-substituted POMs has recently been described. For example, Proust et al. report on the reaction of [Ru(arene)Cl2]2 with K7[α-PW11O39].14H2O to obtain the monomeric species [α-PW11O39{Ru(arene)(H2O)}]5− and the dimeric species [{α-PW11O39{Ru(arene)}}2{WO2}]8− (Inorg. Chem. 2005, 44, 2826-2835). The formation of the dimeric complexes is reported to depend on the bulkiness of the arene ligand and their isolation is described to be difficult. Nomiya et al., Bulletin of the Chemical Society of Japan, 2007, 80, 724-731, conclude that the steric repulsion between the two (arene)Ru2+ fragments in the dimeric species is not significant, the interconversion between the monomeric and the dimeric species is strongly dependent on the pH of the reaction solution rather than the bulkiness of the arene and the use of an in-situ generated POM precursor has an effect.
Moreover, Kortz et al. report on the reaction of [Ru[C6H6)Cl2]2 with [γ-SiW10O36]8− and [γ-GeW10O36]8−, respectively, to yield [{Ru(C6H6)(H2O)}(γ-XW10O36)]4− (X═Si, Ge) (Inorg. Chem. 2006, 45, 8575-8583). Further, the reaction of [Ru(p-cymene)Cl2]2 with the cyclic [H7P8W48O148]33− anion is described to result in [{K(H2O)}3{Ru(p-cymene) (H2O)}4P8W49O186(H2O)2]27− having four {Ru(p-cymene)(H2O)} fragments grafted on the crown-shaped P8W48 precursor (Dalton Trans., 2007, 2627-2630). WO-A-2007/139616 discloses the diruthenium containing POM [Ru2(H2O)6X2W20O70]m− (X═SbIII, BiIII, AsIII, SeIV or TeIV) which has been found to exist in a Krebs-type structure, i.e. a dimeric POM consisting of two trilacunary Keggin fragments B-β-[XW9O33]p− that are linked by two {WO2}2+ and two {Ru(H2O)3}q+ cations.
However, there is still a need for further Ru-containing POMs showing useful properties in homogeneous or heterogeneous catalytic applications.
Therefore, it is an object of the present invention to provide Ru-containing POMs which are useful as catalysts in homogeneous and heterogeneous oxidation reactions of organic substrates. Furthermore, such Ru-containing POMs should be easily and reproducibly prepared in high yield and purity. Moreover, they should be useful as precursors for preparing mixed metal oxide catalysts.